This invention relates to the field of geophysical prospecting and, more particularly, to a method of imaging the subsurface of the earth in depth using compressional-wave, shear-wave and mode-converted wave data.
Seismic data acquired in the field is always recorded in time due to the nature of the acquisition process. For seismic exploration using both compressional and mode-converted waves, it is a common practice to deploy a large number of multicomponent geophones on the surface of the earth and to record the vertical and horizontal vibrations of the earth at each geophone location to obtain a collection of seismic traces. When the vibrations are caused by a seismic source activated at a known time and location, the recorded data can be processed using methods well known in the art to produce an image of the subsurface. The image thus produced is commonly interpreted by geophysicists to detect the presence of hydrocarbons.
The geophones sensitive to vertical vibration modes record mostly compressional (P) waves while the geophones sensitive to horizontal modes record mostly shear (S) waves. The shear wave energy may be due to mode-conversion, i.e., when some compressional energy is converted to shear wave energy at a subsurface layer boundary or other impedance contrast. This type of wave is also referred to as a PS wave, signifying the conversion from P-wave to S-wave. Downgoing P-wave energy reflected as an upgoing P-wave and then recorded is referred to as a PP reflection.
Seismic data are interpreted to provide subsurface information of geological structure and conditions prior to drilling for minerals such as oil and gas. Seismic data are often displayed as xe2x80x9ctime sectionsxe2x80x9d where the vertical scale is linear in arrival time, i.e., an ordinary seismic section. The seismic data interpreted on a time section is converted to depth before drilling for hydrocarbons commences. Depth conversion of time section data requires an accurate velocity model. Another approach is to depth migrate the seismic data before stacking in order to properly position the subsurface geological entities such as faults, channels, etc. The use of prestack depth migration has proliferated in recent years as the computing power has increased and the relative cost has decreased. Processing of mode-converted data is computationally expensive. Depth migration is more attractive than time migration because the common conversion point (CCP) varies with depth due to the asymmetric ray path of the downgoing compressional wave and the upgoing shear wave. The CCP is the common reflecting point where mode conversion from P- to S-waves or vice-versa occur for a dataset. Reciprocity (the conversion point being the same when source and receiver are interchanged) does not apply because of the asymmetric raypath, making lime migration processing more difficult. Thus the processing of mode-converted data in time is much more difficult and can be less accurate than the processing of compressional wave data. However, the depth migration of seismic data requires an accurate velocity model.
The velocity model for depth migration of compressional wave data can be obtained by one of several techniques discussed in the literature (e.g., Kosloff et al. (1996), Meek et al. (1998), Schultz and Canales (1997), and Jones et al. (1998). The migration of mode-converted data also requires knowledge of the shear wave velocity field. D""Agosto and Michelena (1997) developed a tomographic method to obtain P and S wave velocities from mode-converted data.
Mikhailov and Frasier (2000), Bale et al. (1998), and Shoshitaishvili et al. (2000) have discussed the depth migration of mode-converted data. Their methods assume that the depth model and corresponding P-wave velocities are known from the analysis of the P-wave data. The methods describe using the residual moveout of mode-converted waves to update the velocity model and are fairly complex and computationally expensive.
There is a need for a method of updating the S-wave velocity model that is very simple and quickly leads to an acceptable solution for the final velocity model for depth migration of mode-converted data. The present invention satisfies this need.
The present invention provides for a method of processing mode converted seismic data. The method comprises acquiring seismic data and determining a P-wave velocity model for the seismic data. An S-wave velocity model is determined from the P-wave velocity model. The mode converted seismic data are depth migrated forming CIP mode converted seismic data gathers. Corrected S-wave velocities are determined from near-offset data of the CIP gathers which corrects the depth migrated mode converted seismic data to the P-wave velocity model depth. A correction factor is provided and the iterating with the method quickly converges to a solution.
It is an objective of this invention to provide an efficient method of updating the shear wave velocity model for prestack depth migration of mode-converted data. First, a seismic dataset (P-wave) is processed and prestack depth migrated using a number of different methods. A velocity model for P-waves is obtained which is used to calculate the travel-time tables for the downgoing P-waves. This model provides an initial velocity model for S-waves that is used to calculate the travel-time tables for the upgoing S-waves. The travel-time tables calculated from these steps are then used to depth migrate the mode-converted data. The common image point (CIP) depth gathers generated by depth migration are then examined to determine if the S-velocity of a horizon is too low or too high with respect to the S-velocity that flattens the seismic events on the CIP gathers. The depth of the near-offset on the CIP mode-converted image gather for a given horizon is compared to its depth on the velocity model for P-waves at the corresponding surface location. The discrepancy in depth is then used to update the S-wave velocity. The mode-converted data is depth migrated again with the updated S-wave velocity model and the CIP gathers are examined. Generally, this procedure leads to a satisfactory velocity model. However, another iteration of this process may be necessary sometimes to flatten the seismic event on the CIP gathers.